The proposal to build ships of cement created almost as much of a popular sensation as did the first iron boat. Although the public had accepted iron and then steel as a perfectly proper material for shipbuilding, concrete seemed too much like stone and it did not seem possible that artificial stone could be made to float. They did not realize that a cubic foot of steel weighs four times as much as the same volume of concrete. Of course concrete does not begin to have the tensile strength of steel and consequently the walls of a concrete ship must be made relatively thick. For this reason a concrete vessel is heavier than a steel vessel. She draws more water and requires a larger power plant, and because of her greater mass she is not so readily maneuvered. SUBMARINE NAVIGATION As was explained in Chapter VI, a body will float only so long as it is lighter than the volume of water it displaces. It is almost impossible to keep a body suspended in water unless some portion of it is exposed above the surface. If it starts sinking it will keep on going down until it reaches the bottom of the sea. There is a popular notion that at great depths water becomes dense enough to float solid iron, but water is practically incompressible and its density at a depth of five miles is only slightly greater than that at the surface. An object must therefore either float on the surface or sink to the bottom, unless its weight is exactly equal to the difference between the upward pressure of the water under it and the downward pressure of the water above it. Such an ideal balance it is practically impossible to obtain unless the object itself is compressible. How then can a submarine navigate under water without sinking to the bottom? A SUBMARINE OF THE SEVENTEENTH CENTURY The first solution to this problem dates back to the seventeenth century. Doctor Cornelius Van Drebel, a Netherlander, who was a guest at the court of King James I of England, built three submarines between 1620 and 1624. These were rowboats covered over with a water-tight deck and propelled by twelve oarsmen. It is recorded that Van Drebel discovered a means of holding the boats submerged by observing some fishermen towing baskets full of fish up the Thames. The barks to which the baskets were attached by cables were weighted down by the load they were towing, but when the cables slackened the boats rose a little bit. Van Drebel’s method of applying this principle was evidently to attach a weight to the boat which trailed along the bottom. When the oarsmen propelled the boat, she was pulled down under the surface by the drag, but when the rowing ceased the boat would float up to the surface. King James himself is said to have made a journey of several hours’ duration in one these boats, which was kept at a depth of twelve to fifteen feet below the surface. Progress must have been very slow because the range of the submarine was given as five or six miles.